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Yes, that should be possible.

The MacBook Pro has two Thunderbolt 3 controllers, one on each side. Each Thunderbolt 3 controller takes two DisplayPort inputs from the GPU of the MacBook Pro. Both DisplayPort signals can be transmitted down a single Thunderbolt 3 cable.

Each LG UltraFine 5K display requires two DisplayPort signals over Thunderbolt to support 5K 60Hz resolution. This is why each LG UltraFine 5K requires a separate Thunderbolt controller (one on each side of the MacBook Pro). The Thunderbolt controller in the LG UltraFine 5K uses both DisplayPort signals, that is why it does not have a second Thunderbolt 3 port to daisy chain other Thunderbolt devices.

The Blackmagic eGPU has a Thunderbolt 3 controller which takes two DisplayPort inputs from the GPU of the BlackMagic eGPU. Both DisplayPort signals can be transmitted down a single Thunderbolt 3 cable.

Because the GPU of the Blackmagic eGPU is separate from the MacBook Pro GPU, and because the LG UltraFine 5K connected to the Blackmagic GPU takes DisplayPort signals from the GPU of the Blackmagic eGPU, there will be no conflict. This is not possible with other eGPU solutions unless you have a Thunderbolt 3 add-in card (with DisplayPort inputs) between the Thunderbolt display and the GPU of the eGPU.

If the OS doesn't allow it, then that's a bug that should be reported.

The displays connected to the MacBook Pro will not be accelerated like the display connected to the Blackmagic eGPU. They might see some improvement, depending on the software.